1. Field of the Invention
This invention relates generally to devices having both primary and one or more secondary power sources capable of supplying power, and more particularly to an improved test mode structure and method that accurately measures the current draw of such devices.
2. Background of the Invention
Devices having a primary power source and at least one secondary power source, both capable of supplying needed power, rely upon the secondary power source(s), such as a back-up battery, to supply needed power when the primary power source, such as an external power supply, is no longer sufficient to power the device. Such multiple power source devices, hereinafter referred to as xe2x80x9cmulti-power-source devices,xe2x80x9d have become such an indispensable part of technology that it would be difficult to enumerate all of the particular applications and environments in which they are used. Multi-power-source devices are often battery-backed devices, such as static random access memories (SRAMs) and battery-backed controllers and battery-backed power supplies, which are widely used in a variety of applications and environments. As used herein, the term xe2x80x9cbatteryxe2x80x9d refers to any technology capable of storing electrical charge that can be drawn on at some future time by the multi-power-source device in the event that the primary power supply to the device becomes insufficient or fails, and can include capacitors or other electrical storage devices.
Any multi-power-source device, however, is only as reliable as the secondary power source(s) that are providing back-up power to it. In the case of a battery-backed device that relies upon one or more batteries as secondary power sources, batteries are customarily rated, as reflected in their specification sheet, to provide or source a certain amount of current for a defined time period under nominal operating conditions. A battery may thus be guaranteed to source 600 nA of current for ten years at a nominal temperature, for instance. This battery is suitable for a particular device for ten years, then, only if it can be ascertained that the device will demand 600 nA under nominal operating conditions. Unless the current draw of the battery-backed device can be known with certainty, there is no way to guarantee that the particular battery will be sufficient for ten years.
In order to ensure that a particular secondary, back-up power source is sufficient to provide back-up power should it become needed, it is therefore necessary to know with certainty the expected current draw of the device. In many situations, the current draw of a multi-power-source device may readily ascertainable. In other situations, however, this is not the case. Consider, for instance, the current draw of a battery-backed integrated circuit (IC) device. Such IC devices are tested as a matter of course to prove that they will operate in accordance with their specification. A normal part of the IC testing process includes a burn-in step during which the IC device is subjected to voltage and temperature stresses to identify potential weaknesses of the device.
These stresses to which a device is subjected can cause the device to draw more current from a secondary power source or supply than it should. Device leakage can be induced by voltage and temperature stresses or by the stresses necessary to obtain infant-life defects of the device, for instance. The amount of current draw of the device upon the battery or other secondary power source may be difficult to readily measure, particularly in those instances in which the device is an IC device with a back-up battery, the leads of which are not readily accessible to a tester (as in when the back-up battery is attached to the IC after the burn-in step). From the foregoing discussion, it can be seen that there is a need in the art to be able to accurately measure the current of the device even when the battery itself is not readily accessible for testing.
It is accordingly an object of the present invention to ensure accuracy of measurement of the current draw of a multi-power-source device during a test mode of the device.
Therefore, according to the present invention, accuracy of measurement of the current draw of a multi-power-source device during a test mode of the device is ensured by a method and structure that prevents the generation of a variable current of the device during the test mode. The variable current is prevented by causing a primary power supply of the device to be greatly reduced or eliminated in such a manner as to not interfere with the test mode of the device. The test mode of the device is entered and enabled before the current draw of the device can be measured. According to a preferred embodiment, the test mode is entered by providing an elevated voltage to the device, higher than the primary power supply, and the test mode is enabled by setting one or more test bits of one or more data storage elements, such as registers, that minimally allow a control signal provided to the device to ensure that the test bits are not reset during the test mode.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.